Intramolecular [4+2] versus[2+2] cycloadditions in P–X–P-linked biphospholes (X = O, S)

Under solar light at room temperature, the tungsten pentacarbonyl complex of the P-S-P-linked biphosphole (6) undergoes a [2+2] intramolecular cycloaddition. Then, this [2+2] dimer (7) gives the original [4+4] dimer (8) via a Cope rearrangement. This sequence stands in sharp contrast with the behavior of the corresponding P-O-P-linked biphosphole (2) which undergoes a classical [4+2] cycloaddition.